Means for generating sample flames



p 1970 A. A. VENGHIATTIS 3,529,913

MEANS FOR GENERATING SAMPLE FLAMES I Filed Dec. 28, 1967 INVENTIOR. fllens f], 'Vezzy/zmiiwf United States Patent 3,529,913 MEANS FOR GENERATING SAMPLE FLAMES Alexis A. Venghiattis, Ridgefield, Conn, assignor to The Perkin-Elmer Corporation, Norwalk, Comm, a corporation of New York Filed Dec. 28, 1967, Ser. No. 694,156 Int. Cl. F23d 13/36 US. Cl. 431-253 5 Claims ABSTRACT OF THE DISCLOSURE In, for example, atomic absorption spectroscopy, it is desirable to cause a flame (containing an atomized sample material) to go parallelly along an axis without the burner itself crossing or being on this axis (i.e., physically interfering with the sample radiation beam). Although this problem may be solved by existing techniques, such as for example, the use of a deflector (which may for example be in the form of a hollow tube) for reflecting the flame into the desired path, such techniques have a limitation as to both contamination and heat damage of the deflector. The present technique utilizes the discovery that flames (for example from a Bunsen-type burner) will add vectorially when they meet at a moderate included angle at a single point. Thus two or more burner nozzles completely outside of the beam supply angled flames, which meet at a point on the desired axis and have a vectorial resultant along this axis, thereby forming a long flame along the length of the sample radiation beam without physically interfering with the beam. In fact the resultant flame is in general longer than that obtained from any one of the single burners.

This invention relates to burner flames of the type particularly useful for introducing atomized sample mate rials into the sample radiation beam in atomic absorption spectroscopy. More particularly the invention concerns a technique and apparatus for combining at least two flames which are angled relative to the sample radiation beam axis (so that the burners themselves are outside of the sample radiation beam) into a single beam which is parallel to and coincident with the beam axis.

In, for example, atomic absorption spectroscopy, a liquid sample material (transferred for example into the form of a fine liquid mist or vapor) is introduced into a sample flame traversed by a beam of radiant energy, for the purpose of measuring the absorption of at least one spectral line of the radiation by at least one (metallic) component part of the sample in its atomic state. Where the concentration of the metal component in the sample material is relatively high (compared to the sensitivity or detection limit of the spectrometer being used), the conventional technique is merely to cause the sample flame to intersect the sample radiation beam substantially at a right angle. Where the sample material concentra tion is relatively low (or its radiation absorption rela tively weak), the sample flame is conventionally made relatively wide (i.e., transverse to the flame axis) and oriented with the radiation beam so that the large widt of the flame is oriented along the radiation beam axis so as to intercept a substantial length of the beam. In this manner the radiation beam passes through a relatively long dimension of the flame, even though the flame axis (i.e., along the gas flow direction) and the beam axis are substantially perpendicular.

To increase the effective length of the zone (as measured along the radiation beam axis) of intersection of the sample flame and radiation beam, the flame axis itself may be made parallel to (and preferably substantially coincident with) the radiation beam axis. Heretofore this has been accomplished by positioning the burner at a (relatively small) angle to the beam axis and providing a deflector on the opposite side of the beam so as to more or less reflect the flame along the beam. Since as a practical matter the flame never is specularly reflected by the deflector, such deflectors typically are in the form of hollow tubes surrounding the radiation beam, into one 1' end of which the burner flame is introduced at a small angle. In this manner the tubular deflector tends to direct the flame generally along the radiation beam. This prior art technique has various limitations, among which are the difliculty of finding a material for the tube (which may be for example fused silica, pressed graphite or various ceramics) which can withstand repeated exposure to temperatures on the order of 2500 to 3500 C.; further the interior surface of the deflector tube, being exposed to vaporized sample during a particular analysis, tends to be contaminated with previous samples when subsequent analyses of different samples are run.

The present invention is based on the observation that flames from burners (for example, of the Bunsen type) will add in the same manner as vectors when the flames intersect at a point at relativelysmall included angles. Thus two or more flames at converging angles relative to the radiant beam axis and intersecting on the axis may be made to vectorially add to give a resultant long flame coaxial with the radiant beam.

An object of the invention is the provision of a technique and apparatus for causing a long sample flame to extend coaxial with a radiation beam, while obviating the use of physical deflectors for the flame.

Another object of the invention is providing such a long flame coincident with the radiation beam, which flame is actually augmented in length.

Further objects, features and advantages of the invention will be obvious to one skilled in the art upon reading the following detailed description of an exemplary embodiment of the invention in conjunction with the accompanying drawings, in which:

FIG. 1 is a vertical cross section through the center line of an exemplary burner assembly according to the invention, and the radiation beam with which it is used; and

FIG. 2 is a front elevation of the same assembly, seen from the right in FIG. 1, with some parts broken away.

The exemplary embodiment of the invention will be explained in conjunction with its use, for example, for supplying a relatively long sample flame along the radiation beam of an atomic absorption spectrometer. For the purposes of illustration, reference may be made to the various types of atomic absorption spectrometers somewhat schematically illustrated in US. Letters Patent No. 2,847,- 899 to A. Walsh. In each of the: four embodiments in this patent, an atomic vapor of the sample material, including the tested-for metallic element, is introduced into the radiation beam (at 4 in FIG. 1, at 16 in FIG. 2, at 27 in FIG. 3, and at 35 in FIG. 4- of the Walsh patent). In this manner the amount of absorption of the sample radiation beam by the atomized tested-for metallic element (at a characteristic spectral line) may be measured. As noted in the Walsh patent (see column 2, lines 56 57, for example) one conventional technique for introducing an atomic vapor of the sample into the beam is by spraying (or aspirating) a solution of the sample substance into a flame (for example, by premixing the sample solution as a mist with the fuel and oxidant gases supplied to a total consumption burner).

Typically the sample radiation beam cross section is relatively small, while the path length (along the axis of the beam) available for introducing the sample is relatively long. In order to increase the total amount of sample vapor in the radiation beam (and therefore the sensitivity of the instrument) the head of the burner may be elongated in the direction of the beam axis; nevertheless, such elongation of the burner head has a practical limit. Additionally the conventional technique in which the burner flame axis (as measured by the gas flow) is substantially perpendicular to the radiant beam axis is inherently ineificient relative to the total quantity of sample vapor available, since the atomized sample passes through the relatively small diameter of the beam in a relatively short time. In order to increase the sensitivity of the sample absorption when utilizing low concentrations or relatively inefliciently absorbed spectral lines, it is desirable to cause the flame to be more or less parallel to, and substantially coincident with, the sample radiation beam. Obviously a conventional burner could not accomplish this by itself without the burner parts substantially obstructing the sample radiation beam. An existing technique to cause the sample flame to approximate parallelism and coincidence with the beam axis involves surrounding the radiation beam with a tubular element (of slightly larger internal diameter than the beam), and introducing the flame into one end of the tubular element which acts to guide the flame by deflection. Unfortunately the tube tends to be contaminated by the sample material, and, for most materials, the tube will ultimately be destroyed by the heat. For these reasons this is not a particularly convenient nor inexpensive technique, especially When many difierent samples are to be analyzed.

The invention causes the flame containing the vaporized sample to be made substantially coincident to the beam, while avoiding the drawbacks of utilizing such a tubular (or other physical) deflector of the flame. In FIG. 1 the radiant energy beam 20, relatively rich in at least one spectral line which the tested-for element will absorb in its atomic state, may be generated by any conventional spectrally emitting light source (typically a lamp having a hollow cathode, at least partly composed of the tested-for metal). This sample radiant energy beam is assumed to have a central axis 22 and to generally remain within a cylinder (at least in the area of the instrument at which the sample vapor is introduced), indicated by the upper and lower extreme ray 24, 26, respectively. The technique and exemplary apparatus of the invention causes a long sample-vapor-containing flame to be ultimately generated along and substantially within beam 20, by angling a plurality (three in the illustrated device) of flames to a single particular point 32 substantially on the beam axis 20.

In particular the three component flames (34, 36, 38), assumed to be of equal strength, would intersect so as to form the edges of a pyramid having three equal sides (compare FIG. 2), with point 32 being a vertex of the pyramid. Obviously a dilferent number of equal strength flames may be used; in general they would be symmetrically arranged relative to the desired beam axis. For example, two identical flames would meet so as to form an isosceles triangle, the resultant single flame lying along the bisector of the included angle thus formed. It is not even necessary that the flames be identical (although it is preferable both for ease of manufacturing and for minimizing the tendency of a stronger flame to split a weaker one). In general the flames will add in the same manner as mathematical vectors, as they intersect. Thus any arrangement of either equal or different strength flames may be combined at moderate angles (at a 60 included angle, for example) so as to form a vectorially additive resultant flame. In general two similar strength flames will vectorially add (and give a longer resultant) if the included angle at the point of impingement is less than For two component flames of equal magnitude, F, the resultant flame will have a magnitude, R, given by:

i R-2F cos 2 (1a) 11 R/F 2 cos (1b) where i is the included angle between the two component flames, and i/2 is therefore the angle between either of these component flames and the resultant flame. Thus R will be greater than F, if R/F 1. From equation lb above, then:

i 2 cos 1 i 1/ cos 2 2 (2b Since the arc cosine of 60 is /2 and the cosine of angles less than 60 are greater than /2,

i l20 (3b) Thus as long as the included angle between two equal component flames is less than 120, the resulting flame will be longer than the original component flames. Obviously, the smaller the included angle, the larger will be the resultant (for the same original component flames). On the other hand, practical limitations arise from the fact that the burner nozzles (and other parts of the burner assembly) must be completely outside the radiant energy beam, and the fact that the component flames are partially spent in reaching the intersection point (e.g., point 32 in FIG. 1). In other words a compromise must be made between the ideal additive condition (an extremely small included angle between the flames at the intersection point) and the fact that a very small included angle will cause the component flames to meet only after most of the flame has been used up in the long travel to their intersection. In the specific example, each of the three component flames makes an angle relative to both of the other flames of approximately 45, which has been found satisfactory in practice.

FIGS. 1 and 2 thus illustrate a specific embodiment of the invention for the exemplary case in which three equal flames are added. Each of the flames 34, 36 and 38 originate from identical burner nozzles, 44, 46 and 48, respectively. Each of these nozzles has the shape best seen in regard to nozzle 44 in FIG. 1, and including a nozzle end 40, connected by a hollow tubular portion 41 to an integral rounded larger ball seat portion 42. Each of the burner heads is mounted in a main burner head housing 50 of generally apertured disc shape, by being seated in respective threaded apertures 52 in housing '50. Each of the ball seat ends 42 of the various burner nozzles is retained in the respective apertures 52 by a knurled, externally threaded retaining nut 54, having a generally conical relieved interior, as indicated at 56. Thus each of the burner nozzles may be angularly adjusted about its ball seat portion 42 when its knurled retaining nut is loosened and rigidly held in such adjusted position by tightening the nut 54.

The internal tubular aperture or channel 43 of each nozzle communicates as at 45 with an annular channel 60 formed in the main head housing 50 enclosed by an annular disc-shaped cap 62, having a large central aperture at 64 aligned with the central aperture 58 in housing 50. Cap 62 closes in a gas-tight manner the annular channel 60 in housing 50, for example by the use of sealing O-rings 66, 68. Preferably cap 62 is attached to housing 50 in such a manner (as by press fit) that an extraordinary excessive pressure of the gas within annular channel 60 can dislodge cap 62, thereby forming a safety valve. Such an optional provision minimizes the destructive effect of an explosion (i.e., flashback) of the fueloxidant mixture Within annular channel 60. It may be noted that such flashback is highly improbable with this type of burner assembly because the length and small diameter of the nozzle which would choke a flame tending to flash back.

If the burner assembly is to be used with fuel-oxidant mixtures having high thermal output (for example a mixture of acetylene and nitrous oxide), it is preferable to provide auxiliary cooling to the assembly. For such purpose the burner head housing 50 may contain a peripheral channel 70 for circulating cooling fluid (e.g., a liquid such as water) to those parts of the housing adjacent the nozzles (44, 46, 48). A circumferential ring 72 acts as the closure member for peripheral channel 70. Inlet and outlet tubes 74, 76 respectively communicate through ring 72 to the peripheral channel (compare FIG. 2), and may be connected, as by conventional rubber tubing 75, 77 respectively, to a source and a sink of any conventional cooling medium (e.g., water).

The fuel-oxidant mixture containing the sample material (typically as a mist or aerosol) is introduced into the annular channel 60 in housing 50 through a collet 80, screw-threadiingly engaging as at 82 with a threaded portion 84 of housing 50. Collet 80 contains a central tubular aperture 86 and may threadedly engage as at 88 with an internally threaded portion 92 of the burner column 90. The portion 92 may be attached in a gas-tight manner to the generally tubular main member 94 of the burner column as by a recessed O-ring 96 and a set screw 98. Tubular member 94 may be connected in any conventional manner to the mixing chamber of any conventional mixing type of burner (for example, the Premix Burner Assembly, Part No. 303-0110, manufactured by The Perkin- Elmer Corporation, Norwalk, Conn). The fuel, oxidant and aspirated sample solution are pro-mixed in a known manner before entry into tubular member 94.

An alternate technique would be to introduce the sample solution as an aerosol (formed by any conventional means) directly into the resultant flame through a separate nebulizer head, the sample spray being introduced into the flame, for example, at a very small angle relative to the axis of the flame (and to the coaxial radiant energy beam, of course).

The burner assembly illustrated is thus capable of generating a plurality of (in this case three) component flames (34, 36, 38) each of which contains the atomized sample material, and to aim all of the flames to a common point (32) in such manner as to form a single resulting long flame 30 coincident with the radiant beam 20, without any parts of the burner assembly interfering with the path of this radiation beam. Since none of the flames (component flames 34, 36, 38 or the final resulting flame 30) directly contact any material object, there can be no sample contamination build-up (nor heat damage to any such material element).

Optionally, the volume in which the resultant flame 30 is generated may be surrounded by a highly porous, generally cylindrical shield 100 (see FIG. 1). Such a partially open shield may be made from a Wire mesh screen 102 rolled into the form of a hollow cylindrical tube. Not only is this mesh-screen tube open at both its ends, 104, 106, but it is of a sufliciently large diameter as to be completely outside of both the radiant energy beam 20 and the flame 30 (compare FIGS. 1 and 2). Such a screen acts to limit convection currents in the atmosphere (air) surrounding the flame as well as to form a sharp temperature gradient in this atmosphere. Both effects combine to create a channeling or chimney effect so as to cause a further effective increase in the length of the resultant flame 30. This effective lengthening is usually appreciable, and may cause up to a doubling increase) in the length of the flame, as compared to an unshielded flame. It should be noted that such a wire mesh tube does not deflect the flame, nor even contact any part of it. Such a wire mesh screen is neither attacked by the flame nor subject to any substantial sample contamination. Further since it need not be highly heat resistant, tube 100 may be made of inexpensive materials (i.e., be made of a conventional Wire screen material, of the type used for excluding insects); and thus be inexpensively replaced as often as desired.

As stated above, the use of such a porous shield 100 is considered as purely optional, the resultant flame generation being highly satisfactory without it, but even further elongated when such a screening tube is additionally used. Obviously the invention is not limited to the use of any particular number of component flame generators or to the specific details of the exemplary burner assembly illustrated. On the contrary the invention is defined solely by the scope of the appended claims.

What is claimed is:

1. A device for forming a long flame etxending along an existing radiant energy beam, While avoiding physical interference of the burner parts generating said flame With said beam, comprising:

a first burner nozzle means outside of said beam, and directed at and making'a moderate angle with the axis of said beam, whereby the flame generated thereby intersects said beam axis at a particular point,

at least a second burner nozzle means outside of said beam, and directed at said same particular point and making an angle with said beam axis, whereby the flame generated thereby intersects said beam axis and said first flame at said same particular point,

means for supplying to each of said first and second burner nozzle means an already pre-mixed fuel and oxidant gaseous mixture, so that each of the flames generated start substantially immediately adjacent each of said nozzle means,

each of said burner nozzle means identical,

each of said nozzle means being nominally at an equal angle relative to and arranged symmetrically about said beam axis,

the vector addition resultant of the flames from said identical, symmetrically arranged nozzle means there fore being substantially coincident with said beam axis, said vector addition resultant being therefore a long single flame lying along said axis,

each of said nozzle means forming a substantially identical part of the same burner head assembly,

all of said nozzle means being connected to and sup plied by a common input channel,

said burner head assembly comprising a generally discshaped housing, having a central aperture through which said existing radiant energy beam passes,

said burner head assembly further comprising a plurality of mounting means for said burner nozzle means, positioned on one surface of said housing and symmetrically arranged about said central aperture,

each of said substantially identical burner nozzle means being supported by said mounting means in such manner as to be aimed generally at said same particular point on said radiant beam axis,

whereby both the manufacture and adjustment of said burner head assembly, and the ease of making each being substantially flame substantially identical, both in size and in constituency, is simplified.

2. A device according to claim 1, in Which:

each of said nozzle mounting means is adjustable to allow varying the exact aim of each of said nozzle means.

3. A device according to claim 1, in which:

said disc-shaped housing has a substantially annular internal channel, supplied with the pro-mixed fuel and oxidant combustion gases,

and each of said symmetrically arranged nozzle means is communicatingly connected to said annular channel, so as to receive the same combustion gases.

4. A device according to claim 3, in which:

said disc-shaped housing has an additional peripheral channel, extending generally circumferentially about said housing,

and said burner assembly further comprises means for introducing a cooling fluid to said additional peripheral channel,

whereby the various parts of said housing, including said nozzle means, are maintained at moderate temperatures during burner operation.

5. A device for forming a long flame extending along an existing radiant energy beam, While avoiding physical interference of the burner parts generating said flame with said beam, comprising:

a first burner nozzle means outside of said beam, and directed at and making a moderate angle with the axis of said beam, whereby the flame generated thereby intersects said beam axis at a particular point,

at least a second burner nozzle means outside of said beam, and directed at said same particular point and making an angle with said beam axis, whereby the flame generated thereby intersects said beam axis and said first flame at said same particular point,

means for supplying to each of said first and second burner nozzle means an already pre-mixed fuel and oxidant gaseous mixture, so that each of the flames generated start substantially immediately adjacent each of said nozzle means,

said plurality of burner nozzle means being of such relative size and being so angled relative to each other and said beam axis that the Vector addition resultant of said flames lies substantially coincident with said beam axis,

a generally tubular shielding member surrounding said resultant flame and said radiant energy beam, in a substantially coaxial manner,

said shielding member being of a sufliciently large internal diameter to be completely outside both said resultant flame and said radiant energy beam,

the tubular Walls of said shielding member comprising a substantially open mesh structure,

whereby said single resulting flame along said beam axis is further elongated by said mesh shielding members.

References Cited UNITED STATES PATENTS 774,456 11/ 1904 Smith 239420 1,407,871 2/ 1922 Knudsen 2395 44 XR 2,043,867 6/1936 Rava 431350 XR 2,585,133 2/ 1952 Kempthorne 239422 2,847,899 8/1958 Walsh. 3,122,195 2/1964 Kimmel et al. 431-350 XR 3,381,571 5/1968 Vallee et a1. 1,515,112 11/1924 Hisey 263-5 1,676,564 7/1928 Lausen 239417.5 1,973,935 9/1934 Thorsen 431 3,217,779 11/1965 Reed et al. 431175 XR FOREIGN PATENTS 677,591 12/ 1929 France.

FREDERICK L. MATTESON, JR., Primary Examiner H. B. RAMEY, Assistant Examiner 

